Recap:
1st install, mainboard shows a split second of life; system turns on, lights blink and stuff and immediately shuts down.
Checked cabling etc. it seems fine and keep on trying.
Unmounted the mainboard and tried to power the board on "floating" with the result that fans start moving except for cpu fan that works sporadic and optical drives are not responding to their open buttons.
If you ask me, the original installation was faulty and there was a short circuit somewhere around the board.
Seemingly the pwr on switch was placed ok, otherwise he wouldn't had that split second of life, and even with a shorted reset the system should still stay on.
You tried your best to fix it, trying different stuff and trying to boot it over and over again (if the short was due to faulty installation) not knowing ur basically "nuking" your mainboard.
Offcourse this can also be a manufacturing error.
Only two steps left here (one being a question more).
1. when the system turns on now, do you have you welcoming bios beep ?
2. Try as last resort another PSU, very slight possibility that it's half fried due to short circuiting (witch i find unlikely since decent psu's have excellent protection against it).